The present invention is directed to a non-invasive technique for measuring the properties of skin, and more particularly is concerned with the measurement of the viscoelastic properties of the stratum corneum and subjacent viable epidermis layers of the skin as a means for evaluating the effects which aging, disease, various products and climates have upon the skin.
It is desirable to be able to measure various properties of the skin for the purpose of evaluating the efficacy of skin care products. For example, it is useful to know how a cosmetic or moisturizing lotion affects the youthfulness, smoothness or moistness of the skin, or how effective a specific shaving instrument, pre-shave lotion or after-shave lotion is at reducing irritation. However, due to the lack of valid physical measures in the past, clinical studies of product efficacy often depended on visual and tactile grading, for example by moving the fingertips across the skin surface. These measures are necessarily qualitative and the least significant difference that can be detected depends on the evaluator's experience and on the reproducibility of his judgments. An even greater problem arises from intercomparisons and pooled data from tests graded by different individuals; the built-in variability which exists in such product tests requires large, costly studies before high confidence levels about test results can be reached.
Accordingly, it is a general object of the present invention to provide a novel method for providing a quantitative definition of mechanical properties of the skin using a noninvasive, in vivo measurement technique.
Toward this end, research has been conducted to determine mechanical properties of the skin by evaluating its effect upon shear waves. See Potts et al, "The Dynamic Mechanical Properties of Skin In Vivo", J. Biomechanics, Vol. 16, No. 6, pp. 365-372, 1983. By investigating the propagation and attenuation of shear waves at a number of different frequencies, the propagation velocity and damping length for the skin can be determined at each frequency. It has been found that these two properties are sensitive to the degree of moisturization of the skin. Results of tests utilizing the shear wave measurement technique suggest that aged skin has a lower water content than the skin of younger people. See Potts et al, "Changes With Age in the Moisture Content of Human Skin", J. Investigative Dermatology, Vol. 82, No. 1, pp. 97-100, 1984.
While the basic research conducted by Potts et al indicates that it is possible to determine mechanical properties of skin, it is desirable to further improve upon such research. More specifically, propagation velocity and damping length are frequency dependent parameters that do not readily lend themselves to anatomical interpretation for the purpose of evaluating the efficacy of various products. In particular, it is desirable to quantitatively define properties of the skin in terms which can be more clearly associated with elements of human perception.
Therefore, it is a more specific object of the invention to provide such a method which is capable of measuring the properties of the stratum corneum and epidermal layers of the skin in a manner which enables the effects of a skin care instrument or product to be evaluated.
It is a further object of the invention to provide such a method which is capable of measuring the viscoelastic properties of each of the stratum corneum and subjacent viable epidermis individually with the use of shear waves to facilitate the development and evaluation of conditioners that are directed to specific skin problems.
Toward this end, it is a particular object of the present invention to provide a discrete-element mechanical model which describes the skin, and to define the physical properties of the skin in terms of frequency-independent constants related to such model.
It is another object of the present invention to provide quantitative data which can be correlated with human-perceptible qualities such as smoothness and moistness.
It is yet a further object of the invention to provide a novel method of displaying the results of the quantitative analysis of skin properties in such a manner as to be easily understood by the average consumer of skin care products.